The present invention relates to a surface-acoustic-wave bandpass filter, more particularly to a filter structure that improves the lower stopband attenuation of the filter.
Surface-acoustic-wave filters (hereinafter, SAW filters) exemplifying the prior art are described in Japanese Unexamined Patent Applications 10-93381, 10-173469, and 10-294640.
SAW devices in general have an interdigital transducer (IDT) that excites a surface acoustic wave on a piezoelectric substrate. The interdigital transducer can be designed to give the SAW device various characteristics and functions, the filtering function being among the most important. In the past, SAW filters in which surface acoustic waves propagated between two or more interdigital transducers were predominant, but much recent research has focused on SAW filters of the resonator type.
A SAW resonator. has a single interdigital transducer, and may have reflectors to keep surface acoustic waves from escaping from the interdigital transducer. The impedance characteristics of a SAW resonator are quite similar to those of an inductor-capacitor (LC) resonator, so a SAW filter comprising SAW resonators can be designed by classical methods of electrical filter design.
In the past, however, there has been a trade-off between insertion loss in the passband and attenuation in the stopbands, making it difficult to obtain a SAW filter that combines low passband insertion loss with high out-of-band attenuation, particularly in the lower stopband. Further details will be given below.
An object of the present invention is to improve the attenuation in the lower stopband of a SAW filter, without increasing the insertion loss in the passband.
The invented SAW filter has a plurality of SAW resonators with respective interdigital transducers and reflectors, coupled in a ladder network having at least one series-arm. SAW resonator and at least one shunt-arm SAW resonator. The filter also has a plurality of ground patterns, including a separate ground pattern connected to each interdigital transducer of each shunt-arm SAW resonator, and a separate ground pattern connected to each reflector of each shunt-arm SAW resonator. These ground patterns are electrically isolated from one another, and are independently grounded by a plurality of conductors that couple the ground patterns to at least one ground lead.
Independent grounding of the interdigital transducer and reflectors of each shunt-arm SAW resonator is found to improve the attenuation characteristic. of the filter on the low side of the passband.